The present invention relates to a hexagonally crystalline boron nitride powder, referred to as h-BN hereinbelow, capable of exhibiting excellent behavior in sintering and a method for the preparation thereof. More particularly, the invention relates to a powder of h-BN suitable as a starting material of sintered articles of boron nitride and a method for the preparation of such a h-BN powder.
As is known, h-BN is usually a white powder and the structure thereof is lamellar and crystallographically hexagonal like graphite. h-BN has unique characteristics in many respects including, in particular, heat conductivity, electric insulation, corrosion resistance, lubricity, heat resistance, mechanical workability and so on so that powders of h-BN are widely used in a variety of applications where these excellent properties can be fully utilized. For example, h-BN is used in the applications as an additive in plastics, lubricants and the like in the form of a powder and as jigs, electric insulators, molds and the like in the form of a shaped body and composite material.
Various methods have been developed for the preparation of h-BN useful in a wide variety of applications while the major processes currently practiced for the industrial production of h-BN include:
(1) a method by heating a mixture of borax and urea at a temperature of 800.degree. C. or higher in an atmosphere of ammonia gas disclosed in Japanese Patent Publication No. 38-1610;
(2) a method by heating a mixture of boric acid or boron oxide and calcium phosphate in an atmosphere of ammonia gas disclosed in Japanese Patent Publication No. 42-24669; and
(3) a method by heating a mixture of boric acid and a nitrogen-containing organic compound such as urea, melamine, dicyandiamide and the like at a temperature of 1600.degree. C. or higher disclosed in Japanese Patent Publication No. 48-14559.
Several other methods also have been proposed for the preparation of h-BN. For example, Japanese Patent Publication No. 38-22852 teaches (4) a method of vapor-phase synthesis using boron trichloride and ammonia. The applicability of this method, however, is limited to the preparation of a h-BN product of special grade due to the high costs for the starting materials. With an object to improve the above described third method in which well-developed crystalline structure can hardly be obtained and (5) a method is proposed in Japanese Patent Publication 41-18579 according to which the reaction mixture is admixed with a compound of an alkaline earth metal to promote the growth of crystals. Further, Japanese Patent Publication No. 47-26600 proposes (6) a method for upgrading h-BN products in respect of purity in which a crude h-BN prepared from boric acid as the starting material is admixed with an alkali metal and heated at a temperature of 1000.degree. C. or higher.
The above mentioned first method (1) is disadvantageous because the process is complicated due to the indispensable step of washing. Namely, the sodium constituent contained in a large amount in the starting borax begins to evaporate in the form of sodium oxide when the temperature of the reaction mixture is increased to 1000.degree. C. or higher and the thus vaporized sodium oxide is condensed and deposited on the cold part of the reactor while sodium oxide is deliquescent and corrosive to damage the material or the reactor furnance so that the sodium constituent must be removed by washing with water after a step by heating. Similarly, the above described second method (2) also must include a step to remove the calcium phosphate added to the reaction mixture by washing with an acid.
The above described third method (3) has a problem that the purity of the product cannot be high enough unless the reaction temperature is extremely high at 1600.degree. C. or higher. Further, similarly to the third method, the fifth method (5) to promote crystal growth by the addition of an alkaline earth metal compound also includes a step to remove the additive by washing with an acid.
The sixth method (6) for upgrading the h-BN product by the admixture of an alkali metal compound is disadvantageous because the process is very complicate and lengthy including the steps of synthesis of a crude h-BN at a temperature of 800.degree. C. or higher, pulverization of the crude h-BN and admixture of an alkali metal compound, shaping of the mixture and a heat treatment at a temperature of 1000.degree. C. or higher. In this method, the crude h-BN has a purity of only about 80% to be far from a high-purity product and the treatment for upgrading has an effect of removing the residual boron oxide merely by evaporation and not by further nitridation thereof. Accordingly, the content of boron in the starting material is exploited only in a very low percentage so that the method can hardly provide a complete solution of the problem.
Besides, Japanese Patent Publication No. 49-40124 proposes a method for the preparation of a sintered body of boron nitride by a process of hot-pressing of a powder for sintering which is a powder of boron nitride admixed with a borate of an alkaline earth metal. In such a method of hot-press for the preparation of a sintered body by admixing a boron nitride powder with a sintering aid, the starting boron nitride powder has properties identical to those in the conventional products and the method can provide no fundamental solution of the problems since the crystal growth in a high-purity boron nitride powder may cause anisotropy of the sintered body in the course of sintering while low-crystalline boron nitride powders usually have a low purity with a large content of impurities in addition to the sintering aid so that the sintered body has poor characteristics at high temperatures as a consequence.
Thus, a high-purity h-BN can be obtained in the prior art technology only by undertaking a heat treatment at a high temperature or a complicated step of washing with an acid and so on. In addition, a mere upgrading treatment of a h-BN powder in respect of purity is necessarily accompanied by the growth of crystallites while a boron nitride powder having a large crystallite size is not quite satisfactory as a material for the preparation of a sintered body due to the poor sintering behavior of the powder and the low mechanical strengths of the sintered body thereof at high temperatures.